This application relates to fuel cells, and more particularly, the application relates to a separator plate configuration for a fuel cell.
One type of fuel cell includes a proton exchange membrane (PEM) sandwiched between a cathode and an anode. In a PEM fuel cell, a hydrogen-containing fuel and an oxidizer are directed to opposite sides of the membrane, typically by way of reactant passageways and/or gas diffusion layers, as is known. A separator plate between the anode and cathode of adjacent cells prevents commingling of the reactant gases. The separator plate is typically comprised of an anode-side portion and a cathode-side portion, which portions may be made separately and assembled and, for convenience, are often referred to separately as an anode separator plate and a cathode separator plate, respectively. This latter-mentioned convention is employed herein. Product water is formed by an electrochemical reaction on a cathode side of the fuel cell, and the product water must be drawn away from the cathode side of the cell or it will block the passages to the electrochemical reaction sites, known as flooding.
Additionally, in a typical PEM fuel cell having a solid polymer electrolyte membrane, the heat of the electrochemical reaction tends to dehydrate the membrane, thereby increasing electrical resistance and decreasing performance. A critical challenge when operating a fuel cell is to keep the membrane humidified. Typically, makeup water must be provided to the cell in an amount that will prevent the proton exchange membrane from drying out. The makeup water may be provided through external or internal humidification of the reaction gases.
Some systems utilize a porous separator plate, commonly known as a water transport plate (WTP), between adjacent cells. In one configuration, reactant gas flows through channels on one side of each plate and coolant water flows on the other side. The pores in the plate are sized such that the capillary pressure of the water in the pores prevents reactant gas from crossing the plate to the coolant stream, creating a wet seal, yet allows liquid transfer across the plate if subjected to a pressure differential. These porous plates have characteristics, such as bubble pressure and water permeability, that are used to control the flow of water across the plates. Prior art fuel cells using porous separator plates typically utilize plates having the same characteristic relating to bubble pressure and water permeability, and thus also pore size, whether associated with the anode or with the cathode. Bubble pressure and water permeability are inversely related such that they must be balanced when manufacturing WTPs. Typically, both bubble pressure and water permeability are maximized until an acceptable compromise is reached, which can make manufacture difficult and more costly. This is discussed in commonly-owned U.S. Pat. No. 6,197,442 to Gorman.
The separator plates discussed above are typically used in fuel cell systems that rely on total water management (TWM) for cooling, in which coolant is circulated by a pump and pressure differentials are created across the plate. Other types of fuel cells may utilize non-porous (herein termed “solid”) separator plates that prevent the flow of water across the plates. Similar to porous separator plates, solid separator plates provide reactant flow fields on one side and coolant flow fields on the opposite side.
What is needed is a separator plate, or water transport plate, having characteristics that are less costly to achieve while providing desired fuel cell performance.